The invention relates generally to lightwave communication systems and, more particularly, to techniques for controlling the power level of optical signals transmitted in optical line transmission systems.
Retinal and other types of eye injury can occur from inadvertent direct exposure to the optical signals used in present communication systems. Danger is presented by the power and the wavelength of such signals. Generally, these systems operate with signals having relatively high power concentrated in a tiny beam located outside the visible region.
Recent developments in optical networking have only heightened existing safety concerns. For example, optical amplifiers and other optical components are now being developed to drive optical signals to even higher output power levels. Multi-wavelength systems are also a concern because the total optical power in the optical fiber is the sum of the powers of the individual wavelength components. Consequently, optical systems having total output power of 17 dBm or more (e.g., class 3B power levels) are now being realized as a result of advances in optical amplifier and multi-wavelength optical networking technologies.
Because the extent of injury is most likely proportional to the total output power and the time of exposure, it is necessary to quickly shut off or reduce the output power of a network element in the event of a fiber cut, removed connector, or any other discontinuity in the optical path. In fact, this requirement is mandated by the well-known international standard IEC 60825 for optical transmission systems as set forth by International Electro-technical Commission (IEC), see, for example, the IEC publication International Standard IEC 60825, Safety of Laser Products part 1: Equipment Classification, Requirements and User""s Guide, 1997, which is hereby incorporated by reference.
In some prior arrangements, optical power is controlled as a function of the detection of loss of signal (LOS) in the optical traffic channels. For example, upon detection of loss of signal, a controller is used to send messages to an upstream controller which, in turn, causes the upstream optical source to reduce its power level. One of several disadvantages in these types of arrangements is the delay in effecting control as a result of the messaging that must take place, which is typically software-based messaging. Another disadvantage in these prior arrangements is the amount of bandwidth that is used to support the messaging over the supervisory channels. Furthermore, prior control arrangements that are triggered only by the detection of loss of signal in the traffic channels are known to cause unnecessary power shutdowns, which disrupt service and are undesirable to service providers and customers alike.
Reduction of optical signal power supplied by an upstream network element by a prescribed amount is achieved by automatically controlling the output power level of the upstream network element in response to the detection of both loss of signal power and loss of supervisory signal power at a downstream network element as a result of a fiber cut, open connector, or other potentially hazardous discontinuity. More specifically, by separately controlling the supervisory signal and traffic signals being transmitted from the downstream network element to the upstream network element, control of the upstream network element can be effected without the software-based supervisory messaging which is used in prior arrangements. Moreover, triggering control as a function of the detection of loss of signal in both the traffic and the supervisory channels according to the principles of the invention prevents unnecessary power shutdowns, thus providing a more reliable and robust control scheme as compared with prior arrangements.
In one illustrative embodiment, an optical line system includes two or more network elements, each including optical amplifiers and associated supervisory and control units. In a typical configuration, the network elements are connected so that an optical amplifier in each network element supports transmission of optical signals in one direction along a first optical fiber and another optical amplifier in each network element supports transmission of optical signals in the opposite direction along a second optical fiber. As a result of a cut in the first optical fiber, a loss of signal in the traffic channels and a loss of supervisory signal is detected at the input of the downstream or receiving optical amplifier in the first optical fiber. In response thereto, the optical amplifier transmitting toward the upstream network element via the second optical fiber is shut down. Because the second optical fiber path is not actually cut, the supervisory signal is still being supplied along the second optical fiber path even in the absence of the traffic channels. However, the supervisory signal being transmitted to the upstream network element is then shut down for a predetermined period of time. By shutting off the supervisory signal in the second optical fiber path for a predetermined period of time, a fiber cut is effectively emulated within the second optical fiber path, i.e., loss of signal power (by shutting off the optical amplifier) and loss of supervisory signal (by appropriately controlling the supervisory unit). In response to the detection of loss of signal power and loss of supervisory signal power at the input of the upstream optical amplifier, the upstream optical amplifier is then shut-down and the hazardous condition on the first optical fiber is removed.